Drug delivery devices have application, where persons without formal medical training need to administer an accurate and predefined dose of a medicinal product, such as heparin or insulin. In addition, the medicinal product may be administered on an irregular basis over a short-term or a long-term period.
User operated drug delivery devices are well-known within the medical field.
WO 96/26754 A2 shows a mechanism for accurate dispensing of pre-set quantities of medicinal products from a drug delivery device wherein a rotatable plunger has a number of parallel racks on its internal surface each of which comes into engagement with the first tooth wheel as a piston rod rotates when a dose is set. The device has a second tooth wheel that rotates with the first tooth wheel thus driving a thrust rod into the drug delivery device when the selected dose is delivered.
A user, for instance a patient, may inject a fluid into its body by selecting the pre-set dose. For that purpose, the user moves the button of the drug delivery device by a first movement to pre-set the dose to be administered. With a second movement of the dose button, the fluid is dispensed and may be injected into the patient's body. The procedure can be repeated upon the patient's discretion.
Depending on the medicinal product to be administered, a single dose must be very accurate and the amount of fluid shall not differ between different doses. As the mechanical parts of the drug delivery device comprise some tolerances in between, the device is primed before the first usage. During the priming procedure, all mechanical parts of the drive mechanism of the drug delivery device are initialized and any backlash as well as tolerances between different mechanical parts is reduced.
However, a backlash may occur and tolerances between the different parts of the drive mechanism may increase during the dispensing of several doses. Accordingly, the accuracy of amounts of doses to be dispensed will decrease again. In summary, there is still a need for drug delivery devices with improved dose accuracy during usage.